Surgical stapling instruments used for substantially sequentially applying parallel, linear rows of staggered staples through compressed living tissue are well known in the art, and are commonly used for closure of tissue or organs prior to transection or resection, and for occlusion of organs in thoracic and abdominal plasty procedures. Surgical staplers of this type may be used during an open surgical procedure where an incision is made to provide access to the surgical site, or during a laparoscopic surgical procedure where tissue stapling is accomplished through a generally cylindrical access tube.
Surgical staplers which substantially sequentially fire staples typically comprise a staple housing for enclosing the staples prior to their formation, a pusher having a cam surface, and staple drivers substantially adjacent each staple. As used herein, when it is said that a surgical stapler "substantially sequentially" fires the staples in a linear row, it is meant that the stapler completes the application of some staples in a row before the application of other staples in the row, as opposed to a stapler which generally simultaneously fires all of the staples in a row. In a stapler which "generally simultaneously" fires staples, during at least a portion of the movement of its firing mechanism, all of the staples are in motion relative to the staple housing. The circular stapler disclosed in U.S. Pat. No. 4,754,909 generally simultaneously fires a circular array of staples, and the stapler described in EPO Application No. 514 139 to Solyntjes et al. generally simultaneously fires a plurality of linear, parallel rows of staggered staples.
In a stapler which substantially sequentially fires staples in a linear row, the firing mechanism typically comprises a pusher, staple drivers generally adjacent staples in a staple housing and an anvil. The pusher is movable relative to the staple housing in a firing direction. On its leading edge, the pusher has a camming surface situated at an acute, included angle with the firing direction. The staple drivers have cam follower surfaces for engaging the camming surface to move the staple drivers in a staple formation or staple driving direction which is typically perpendicular to the firing direction. Movement of the staple drivers in the staple formation direction ejects the staples from the staple housing and presses the ejected staples against specially shaped surfaces on the anvil to engage, form and close the staples in tissue between the staple housing and anvil.
In an unformed condition, staples used in a sequentially fired stapler typically comprise a backspan and a pair of legs projecting from the backspan that each include a sharp tissue penetrating surface. During formation of an individual staple, the tips first pass through tissue and then engage the specially shaped surface of the anvil. The force required to initially buckle or bend the staple legs when they engage the anvil is relatively greater than the force required to pierce tissue.
When properly formed, the staples assume a substantially "B" shaped configuration. Improperly or only partially formed staples may result in a variety of adverse consequences for a patient, such as inadequate hemostasis, excessive bleeding or a weakened staple line which could result in dehiscence of the anastomosis or leakage.
In the present context, the phrase, "formation force" pertains to the force required to apply a staple. The phrase "initial maximum formation force" refers to the initial maximum force encountered during formation of a staple which corresponds to the initial buckling or bending of the legs. During the formation of staple loops, a second maximum formation force is encountered that is also substantially greater than the force required to penetrate the tissue. The second maximum formation force may be greater than the initial maximum formation force. The second maximum formation force corresponds to the formation of the legs into loops after the buckling of the legs, but before the legs engage each other or the backspan, or before the final formation of the legs into loops.
It is also believed that a third maximum formation force may be encountered during the final formation of the staple. The third maximum formation force is believed to correspond to staple legs engaging either each other or the backspan, or to the increasing bending forces encountered by the buckling of the staple legs on an ever shortening effective beam length. The third maximum formation force is also relatively greater than the maximum force required to pierce tissue. Graphs of the firing force versus pusher displacement are found in U.S. Pat. Nos. 3,494,533 and 4,767,044, but these graphs do not illustrate the third maximum formation force that was discovered by applicants and mentioned above.
FIG. 42 is a graph of the formation force curve in pounds versus the pusher stroke in inches for a prior art titanium staple which was slightly overcrimped in simulated thin tissue. The staple was a staple designed generally for use in a stapler as shown in EPO Application No. 514 139 to Solyntjes et al. Each of the first, second and third maximum formation forces are referenced as #1, #2 and #3.
The prior art is replete with mechanisms designed to reduce the overall formation force experienced by the surgeon in firing all of the rows of staples in the stapler. For example, U.S. Pat. No. 3,499,591 illustrates a stapler with pusher devices staggered so that peak forces for staples are not simultaneously encountered. European Patent Application No. 545029 discloses further attempts to reduce the operative effort.
A general analysis of the relationship between the pusher, pusher driver and anvil reveals that by reducing the angle between the camming surface of the pusher and the firing direction (or conversely, increasing the angle between the camming surface of the pusher and the staple driving direction), the force encountered by the surgeon may be reduced. U.S. Pat. Nos. 3,079,606 to Bobrov et al. and 3,315,863 to O'Dea illustrate sequentially fired staplers with pushers having camming surfaces at small included angles with the firing direction. The illustrated angles appear to be less than about twenty (20) degrees.
A distal or nose portion for the staple housing is required to house the distal portion of the pusher while the proximal portion of the pusher completes the firing of the distalmost staple in a staple row. When the pusher camming surface forms a shallow angle with the firing direction, the distal or nose portion of the staple housing is relatively lengthy. Such staplers encounter problems when used in a surgical procedure which requires the stapler to staple tissue in a remote position which is not readily accessible to a surgeon, as such staplers require relatively lengthy distal end portions (or "noses") of the staple housing to accommodate the small angled pusher. Examples of such procedures include deep pelvic or thoracic cavity procedures where space is a limiting factor.
The distal end portion of a stapler may limit the access of the stapler to the tissue to be stapled. For example, tissue such as bone or adjacent blood vessels may prevent proper placement of such a stapler on tissue. A stapler with a lengthy distal end portion is also believed to be difficult to maneuver in cramped or tight spaces at least in part due to the lengthy distal end portion.
These problems are only exacerbated when the sequentially fired stapler comprises a laparoscopic stapler, as the surgeon's access to the tissue to be stapled is even further restricted due to the access tube. It is particularly important in laparoscopic surgery to provide a cartridge as small as possible in order to maximize the maneuvering room between the distal end of the access tube and the tissue to be stapled.
U.S. Pat. No. 4,596,351 to Fedotov et al. discloses a stapler having a pusher with a curvilinear camming surface. The angle between the curvilinear camming surface of the pusher of Fedotov et al. and the firing direction constantly changes rendering it difficult to accurately predict the effective angle encountered during the various stages of staple formation.
It is also noted that the pushers of the GIA - 60 surgical stapler (generally available from U.S. Surgical Corporation) appear to comprise first and second linear camming surfaces, but do not include a third linear camming surface. This stapler is generally designed for use in laparoscopic surgery.
Other approaches to the problem of reducing the firing force encountered by a surgeon comprise surgical staplers that are manually fired but include a) a mechanism for providing a mechanical advantage, or b) a powered instrument which utilizes stored energy (such as gas stored in a cylinder). Staplers with mechanical advantage comprise the 3 cm Endostapler known as the Endo GIA-30, available from U.S. Surgical Corporation of Norwalk, Conn. and the 6 cm Endostapler known as the Endopath Linear Cutter 60 available from Ethicon, Inc. of Somerville, N.J. However, these types of staplers are expensive to manufacture, complex and do not provide a surgeon with direct feedback as to how the position of the firing lever travel relates to the length of tissue that has been stapled (and optionally cut).
Another approach to the problem of reducing the firing force experienced by a surgeon is shown in U.S. Pat. Nos. 5,083,695 and 5,141,144 the entire contents of which are herein expressly incorporated by reference. FIG. 18 illustrates a problem overcome by these types of staplers. The stapler 5 illustrated in FIG. 18 comprises a stapler substantially as shown in U.S. Pat. No. 4,863,088. With that stapler 5, the surgeon must push with enough force to overcome not only the firing force of the staples and frictional drag of the pushers and drivers, but also the frictional side or binding load created by pressing on the knob 6 at a location spaced from the axis 4 of the firing rod 11. The binding load is shown in FIG. 18 as the moment M.sub.F and is described by the following equation: EQU M.sub.F =F.sub.F H EQU where: EQU F.sub.F =firing force (pounds); EQU and EQU H=off center height (the distance between the point where the force F.sub.F is applied on the knob 6 and the axis 4 in FIG. 18).
Notably, since the knob 6 is mounted on the side of the stapler 5, when a surgeon presses on the knob 6 in a convenient manner, a moment is created about the firing rod 11. If the moment is large enough, it may even cause the pushers to engage the staple housing increasing the friction encountered by the firing assembly of the device. Thus, it can be seen that the moment may increase the firing force encountered by a surgeon.
The staplers shown in U.S. Pat. Nos. 5,083,695 and 5,141,144 have a firing handle body capable of being fired by simultaneously pressing on both sides of a firing button. The firing button may be located on both sides of the stapler to assist in eliminating any appreciable moment M.sub.F. However, in order to fire the stapler in this manner, surgeons will use both hands. The surgeon should also push equally on both sides of the firing button to avoid a resultant moment on the firing rod. Such a technique is inconvenient during a laparoscopic surgical procedure where typically only one of the surgeon's hands is available for firing the stapler.
Another firing force issue arises when the surgical stapler is designed to apply six parallel rows of staples as opposed to the typical stapler which applies only four rows of staples. For example, in laparoscopic surgery where hemostasis and air leakage of lung tissue are particularly important, it may be desirable to add the fifth and sixth rows of staples.
FIG. 16 schematically illustrates a prior art, six row staple pattern applied by the laparoscopic GIA stapler available from U.S. Surgical of Norwalk, Conn. However, if substantially parallel pushers are used to apply that arrangement of staples, first 2, then 4, then 2, then 4 staples are applied. This may lead to a "chatter" problem or a "bumpy" feel to the instrument as peak firing forces fluctuate considerably. Assuming the pushers of the stapler are not staggered, it also requires the surgeon to exert a formation force sufficient to simultaneously form four staples.